It is an objective of the industry to provide lubricating oil compositions which exhibit improvements in minimized engine deposits and low rates of lubricating oil consumption, particularly in diesel engine vehicles.
Among the conventionally used lubricating oil additives, zinc dihydrocarbyl dithiophosphates perform multiple functions in the motor oil, namely, oxidation inhibition, bearing corrosion inhibition, and extreme pressure/antiwear protection for the valve train.
Early patents illustrated compositions using polyisobutenylsuccinimide dispersants in combination with zinc dialkyldithiophosphates which were employed in lubricating oil compositions with other conventional additives such as detergents, viscosity index improvers, rust inhibitors and the like. Typical of these early disclosures are U.S. Pat. Nos. 3,018,247, 3,018,250 and 3,018,291.
Since phosphorus is a catalyst poison for catalytic converters, and since the zinc itself offers a source for sulfated ash, the art has sought to reduce or eliminate such zinc-phosphorus-containing motor oil components. Exemplary of prior art references directed to the reduction in phosphorus-containing lubricant additives are U.S. Pat. Nos. 4,147,640; 4,330,420; and 4,639,324.
U.S. Pat. No. 4,147,640 relates to lubricating oils having improved antioxidant and antiwear properties which are obtained by reacting an olefinic hydrocarbon having from 6 to 8 carbon atoms and about 1 to 3 olefinic double bonds concurrently with sulfur and hydrogen sulfide and thereafter reacting the resulting reaction intermediate with additional olefin hydrocarbon. These additives are disclosed to be generally used in conjunction with other conventional oil additives such as overbased metal detergents, polyisobutenylsuccinimide dispersants, and phenolic antioxidants. While it is disclosed that the amount of the zinc additive can be greatly reduced, giving a "low ash" or "no ash" lubricant formulation, it is apparent the patentee was referring to Zn-derived ash, and not total SASH levels.
U.S. Pat. No. 4,330,420 relates to low ash, low phosphorus motor oils having improved oxidation stability as a result the inclusion of synergistic amounts of dialkyldiphenylamine antioxidant and sulfurized polyolefin. It is disclosed that the synergism between these two additives compensates for the decreased amounts of phosphorus in the form of zinc dithiophosphate. The fully formulated motor oils are said to comprise 2 to 10 wt. % of ashless dispersant, 0.5 to 5 wt. % of recited magnesium or calcium detergent salts (to provide at least 0.1% of magnesium or calcium), from 0.5 to 2.0 wt. % of zinc dialkyldithiophosphate; from 0.2 to 2.0 wt. % of a dialkyldiphenolamine antioxidant; from 0.2 to 4 wt. % of a sulfurized polyolefin antioxidant; from 2 to 10 wt. % of a first, ethylene propylene VI improver; from 2 to 10 wt. % of a second VI improver consisting of methacrylate terpolymer, and the balance baseoil.
U.S. Pat. No. 4,639,324 discloses that metal dithiophosphate salts, while useful as antioxidants, are a source of ash, and discloses an ashless antioxidant comprising a reaction product made by reacting at least one aliphatic olefinically unsaturated hydrocarbon having from 8 to 36 carbons concurrently with sulfur and at least one fatty acid ester to obtain a reaction intermediate which is then reacted with additional sulfur and a dimer of cyclopentadiene or lower C.sub.1 to C.sub.4 alkyl substituted cyclopentadiene dimers. It is disclosed that these additives in lubricating compositions are generally used in conjunction with other conventional oil additives such as neutral and overbased calcium or magnesium alkaryl sulfonates, dispersants and phenolic antioxidants. It is disclosed that when using the additives of this invention, the amount of the zinc additive can be greatly reduced giving a "low ash" or "no ash" lubricant formulation. Again, it is apparent that the patentee was referring to Zn-derived ash, and not to total SASH.
Metal detergents have been heretofore employed in motor oils to assist in controlling varnish formation and corrosion, and to thereby minimize the adverse impact which varnish and corrosion have upon the efficiency of an internal combustion engine by minimizing the clogging of restricted openings and the reduction in the clearance of moving parts.
U.S. Pat. No. 4,089,791 relates to low ash mineral lubricating oil compositions comprising a mineral oil base in minor amounts of an overbased alkaline earth metal compound, a zinc dihydrocarbyl dithiphosphate (ZDDP) and a substituted trialkanolamine compound, wherein at least 50% of the ZDDP compounds consists of zinc dialkaryl dithiophosphates, in order to provide a formulated motor oil which will pass the MS IIC Rust Test and the L-38 Bearing Weight Loss Test. The patent illustrates three oil formulations, containing overbased calcium detergent, ZDDP, trialkanolamine and unspecified conventional lubricating oil additives to provide viscosity index improvement, antioxidant, dispersant and anti-foaming properties. The illustrated formulations each had about 0.66 wt. % SASH levels, based on the reported Ca and Zn concentrations. No diesel motor oil formulations are illustrated.
U.S. Pat. No. 4,153,562 relates to antioxidants, which are disclosed to be particularly useful for compounded lubricating oils that are intended for heavy duty use in automotive crankcase formulations of relatively low ash content, wherein the antioxidants are prepared by the condensation of phosphorodithioates of alkylphenol sulfides with unsaturated compounds such as styrene. The antioxidants are exemplified at levels of from 0.3 to 1.25 wt. % in lube oil compositions (Example 3) which also contain about 2.65 wt. % (a.i.) borated polyisobutenylsuccinimide dispersant, about 0.06 wt. % Mg as overbased magnesium sulfonate detergent inhibitor, and about 0.10 wt. % Zn as zinc dialkyldithiophosphate antiwear agent (containing mixed C.sub.4 /C.sub.5 alkyl groups).
U.S. Pat. No. 4,157,972 indicates that the trend to unleaded fuels and ashless lubricating compositions has necessitated the search for non-metallic (ashless) substitutes for metallo-organo detergents, and relates to tetrahydropyrimidyl-substituted compounds which are disclosed to be useful as ashless bases and rust inhibitors. The Examples of the Patent compare the performance of various lubricating oil formulations in a Ford V8 varnish test (Table I) and additional formulations, which are named as either "low-ash" or "ashless", in a Humidity Cabinet Rust Test (Table II). The SASH levels of the "low ash" formulations are not reported and cannot be determined from the information given for the metal detergent- and ZDDP- components.
U.S. Pat. No. 4,165,292 discloses that overbased metal compounds provide effective rust inhibition in automotive crankcase lubricants and that in the absence of overbased additives, as in ashless oils, or when such additives are present in reduced amounts, as in "low ash" oils, rusting becomes a serious problem. Such rust requirements are evaluated by ASTM Sequence IIC engine-tests. The Patent discloses a non-ash forming corrosion or rust inhibitor comprising a combination of an oil-soluble basic organic nitrogen compound (having a recited basicity value) and an alkenyl or alkyl substituted succinic acid having from 12 to 50 carbon atoms. The basic organic nitrogen compound and the carboxylic acid compound are required to be used together to achieve the desired rust-inhibiting properties. It is disclosed that best results are achieved by use of an excess of amine over that required to form the neutral salts of the substituted succinic acid present.
U.S. Pat. No. 4,502,970 relates to improved crankcase lubricating oil compositions containing lubricating oil dispersant, overbased metal detergent, zinc dialkyldithiophosphate antiwear additive and polyisobutenylsuccinic anhydride, in recited amounts. Exemplary lubricating oil formulations are disclosed containing 3 wt. % polyisobutenylsuccinimide dispersant, polyisobutenylsuccinic anhydride, overbased metal sulfonate or overbased sulfurized phenate detergents and zinc dialkyldithiophosphate antiwear agents, in base oil, in amounts of 3.0, 3.0, 2.0, 1.0 and 91.0 wt. %, respectively.
European Patent 24,146 relates to lubricating oil compositions containing copper antioxidants, and exemplifies copper antioxidants in lubricating oil compositions also containing 1.0 wt. % of a 400 TBN magnesium sulphonate (containing 9.2 wt. % magnesium), 0.3 wt. % of a 250 TBN calcium phenate (containing 9.3 wt. % of calcium) and a zinc dialkyldithiophosphate in which the alkyl groups or a mixture of such groups having between 4 and 5 carbon atoms and made by reacting phosphorous P.sub.2 S.sub.5 with a mixture of about 65% isobutyl alcohol and 35% of amyl alcohol, to give a phosphorous level of 1.0 wt. % in lubricating oil composition.
Published British Patent Application 2,062,672 relates to additive compositions comprising sulfurized alkyl phenol and an oil soluble carboxylic dispersant containing a hydrocarbon-based radical having a number average molecular weight of at least 1300, which is disclosed in combination with ash-producing detergents.
However, it is extremely difficult to translate lube oil developments intended for passenger car and light truck service, whether gasoline or light duty diesel engines, into lubricating oils intended for use in heavy duty diesel service.
R. D. Hercamp, SAE Technical Paper Series, Paper No. 831720 (1983) reports development work on engine test procedures to measure the relative ability of various lubricant formulations to control oil consumption in heavy duty diesel engines. The author indicates that lab analysis of crown land deposits on the diesel engine pistons show an organic binder to be present which contains high molecular weight esters, and the author speculates that oxidation products in the oil may be precursors for the binder found in the deposits. It is indicated that improved antioxidants could be the key to prevent premature oil consumption.
A. A. Schetelich, SAE Technical Paper Series, Paper No. 831722 (1983) reports on the effect of lubricating oil parameters on PC-1 type heavy duty diesel lubricating oil performance. It is noted that over the past 30 years, the trend in heavy duty diesel oil industry has been to decrease the sulfated ash levels from 2.5 wt. % sulfated ash (SASH) in 1960 to the typical North American SASH level of 0.8 to 1 wt. %, and to correspondingly decrease the HD oils total base number (TBN) D2896 values from over 20 to the present typical North American TBN values of from 7 to 10. Such reductions in SASH and TBN levels are attributed by the author to be due to improvement in performance of ashless components, including ashless diesel detergents and ashless dispersants. In diesel engine tests, no significant correlation was seen between the level of either piston deposits or oil consumption and the SASH or TBN levels, for about 1% to 2% SASH levels and about 8 to 17 TBN levels. In contrast, a significant correlation was seen between the level of ashless component treat and the amount of piston deposits (at the 92% confidence level) and oil consumption (at the 98% confidence level). It is noted by the authors that this correlation is drawn with respect to diesel fuels having average sulfur levels of less than about 0.5%. It is indicated that the level of buildup of ash is accelerated in the hotter engine areas. The author concludes that at the 97% confidence level there should be a correlation between oil consumption and piston deposits, especially top land deposits, which are believed to contribute to increased oil consumption due to two phenomena: (1) these deposits decrease the amount of blow-by flowing downwardly past the top land, which results in a decreased gas loading behind the top ring of the piston, which in turn leads to higher oil consumption; and (2) increased bore polishing of the piston cylinder liner by the top land deposits which in turn contributes to higher oil consumption by migration of the oil into the firing chamber of the cylinder along the polished bore paths. Therefore, the Paper concluded that reduced ash in the oil should be sought to reduce top land deposits, and hence oil consumption.
This 1983 Schetelich paper reports formulation of 2 test oils, each containing about 1% SASH and having TBN levels of 10 and 9, respectively, wherein each formulated oil contained overbased metal detergent together with a zinc-source.
J. A. McGeehan, SAE Paper No. 831721, pp. 4.848-4.869 (1984) summarized the results of a series of heavy duty diesel engine tests to investigate the effect of top land deposits, fuel sulfur and lubricant viscosity on diesel engine oil consumption and cylinder bore polishing. These authors also indicated that excessive top land deposits cause high oil consumption and cylinder bore polishing, although they added that cylinder board polishing is also caused in high sulfur fuels by corrosion in oils of low alkalinity value. Therefore, they concluded that oil should provide sufficient alkalinity to minimize the corrosive aspect of bore polishing The authors reported that an experimental 0.01% sulfated ash oil, which was tested in a AVL-Mack TZ675 (turbocharged) 120-hour test in combination with a 0.2% fuel sulfur, provided minimum top land deposits and very low oil consumption, which was said to be due to the "very effective ashless inhibitor". This latter component was not further defined Further, from the data presented by the author in FIG. 4 of this Paper, there do not appear to be oil consumption credits to reducing the ash level below 1%, since the oil consumption in the engine actually rose upon reducing the SASH from 1 to 0.01%. This reinforces the author's view that a low, but significant SASH level is required for sufficient alkalinity to avoid oil consumption as a result of bore polishing derived from corrosive aspects of the oil.
McGeehan concluded that the deposits on the top land correlate with oil consumption but are not directly related to the lubricant sulfated ash, and commented that these deposits can be controlled by the crankcase oil formulation.